The present invention is relevant to U.S. patent application Ser. No. 09/941,752 being filed by Daisuke Suzuki, Atsushi Ninomiya, Tsuyoshi Miyashita, Akihito Kandori, Keiji Tsukada and Kouich Yokosawa, and assigned to the present assignee, based on Japanese Patent Application No. 2000-334921 filed on Oct. 30, 2000, and is relevant to U.S. patent application Ser. No. 09/940,507 being filed by Kouichi Yokosawa, Daisuke Suzuki, Keiji Tsukada, Tsuyoshi Miyashita and Akihiko Kandori, and assigned to the present assignee, based on Japanese Patent Application No. 2001-044426 filed on Feb. 21, 2001. In particular, the biomagnetic field measuring apparatus of the invention was developed based on the Instrument For Measuring Magnetic Field as disclosed in JP Pat. App. No. 2000-334921, and the SQUID magneto-meters of the invention are based on the Detection Coil-Integrated Gradiometer And Magnetic Field Measuring Instrument as disclosed in JP Pat. App. No. 2001-044426, which are incorporated herein by reference.
The present invention relates to a biomagnetic field measuring method and apparatus for measuring a biomagnetic field generated by neural action in a brain, myocardial action in the heart of a living body by means of a plurality of fluxmeters including a high-sensitive superconducting quantum interference device (SQUID).
Heretofore, a measured result of a biomagnetic field is represented by a time changing waveform of measured magnetic field components or an iso-magnetic field map prepared by connecting points where the intensity of the magnetic field at arbitrary time is identical. For example, it is known that Z components (Bz) in the orthogonal coordinates or equal-diameter components (Br) in the polar coordinates are measured and values of Bz or Br are expressed as an iso-magnetic field map (H. Hosaka and D. Cohen, J. Electrocardiol., 9-4, 426 (1976)). Further, it is also known that tangential components (Bx, By) in the orthogonal coordinates are measured to be expressed as an iso-magnetic field map for each component or two-dimensional magnetic field vectors are calculated from {square root over ( )}{(Bx)2, (By)2} to be expressed as an iso-magnetic field map (K. Tsukada et al., Review of the Scientific Instruments, 66, 10 (1995)). In addition, a method is known in which normal components Bz are measured and magnetic field components equivalent to tangential components (Bx, By) are analytically calculated from the normal components Bz (T. Miyashita et al., Proceedings 20th International Conference IEEE/EMBS (Hong Kong), 520-523 (1998)).
Heretofore, the analytical result of the biomagnetic field components is represented by using a time waveform of a magnetic field and an iso-magnetic field map. Further, positions, intensities, directions and the like of current sources in a living body at arbitrary time are presumed by solving an inverse problem and these presumed data are used to presume a pre-excited location of arrhythmia in the heart, foci of epilepsy in the brain and the like. In order to trace dynamic phenomena in a certain time zone such as excitation conduction process of myocardium in the heart and neural excitation conduction in the brain, a lot of iso-magnetic field maps at individual time are displayed side by side or loci of vectors of current sources presumed at individual time are represented in a diagram (N. Izumida et al., Japanese Heart Journal, 731-742 (1998)).
It is an object of the present invention to provide biomagnetic field measuring method and apparatus capable of quantifying conduction process of electro-physiological excitation without presumption of a dipole (magnetic field source) and display of many iso-magnetic field maps.
Without arranging many iso-magnetic field maps side by side to analyze dynamic excitation conduction in the heart and the brain by means of the pattern recognition, a graph or diagram representation for quantifying dynamic excitation conduction without using the pattern recognition is requested. A method of presuming current sources every moment can presume current sources as dipole models when the current sources are positioned locally, while generally the current sources are distributed widely with the spread in many time zones. When the inverse problem is solved every moment, many arithmetic operations are required until the solution is converged. Particularly, when the coincidence of a calculated distribution of magnetic fields prepared by presumed current sources and a distribution of actually measured magnetic fields is bad, presumed values of the current sources are deteriorated. Consequently, when the current sources are presumed every moment in a certain time zone, there is a problem that presumption error is increased to thereby produce an analytical result having interrupted continuity in change of time with respect to positions, intensities and directions of the current sources.
In the present invention, the orthogonal coordinates (x, y, z) (magnetic field components are Bx, By and By) and the polar coordinates (r, xcex8, xcfx86) are used as coordinates in measurement of a biomagnetic field. When an object to be measured is the heart, the orthogonal coordinates employing the chest as an xy plane is used. When an object to be measured is the brain, the polar coordinates (r, xcex8, xcfx86) (magnetic field components are Br, Bxcex8 and Bxcfx86) is used since the head has a shape near to a sphere. The magnetic field components (normal components) vertical to the surface of the head are represented by Bz and Br and components (tangential components) parallel to the plane tangential to the surface of the living body are represented by Bx, By, Bxcex8 and Bxcfx86.
The following description is made by using the orthogonal coordinates (x, y, z) by way of example, while when the polar coordinates (r, xcex8, xcfx86) is used, Bz, Bx and By are to be replaced by Br, Bxcex8 and Bxcfx86, respectively.
In the biomagnetic field measuring apparatus of the present invention, a set of sensor arrays is used to measure a biomagnetic field in various different directions. At this time, in order to analyze measured results of the biomagnetic field in many directions, (1) simultaneously with measurement of the biomagnetic field in respective directions, any of an electrocardiograph, a phonocardiograph, a polygraph, an electroencephalograph and the like is used as a living-body signal measuring apparatus to measure and collect living-body signals periodically generated except the biomagnetic field signals and including any of waveforms in electrocardiogram, heart sound, polygraph, electroencephalogram and the like as pairs with the biomagnetic field signals, or (2) synchronous signals synchronizing with the start of application of any stimulation signals generated by stimulating a nervous system by electrical stimulation of part of the living body by means of an electric stimulator, by stimulating auditory nerve by generation of sound by means of an auditory stimulator, by stimulating rhinencephalon by generation of smell by means of a smell stimulator, by stimulating visual area by generation of light signal or color signal by means of a visual stimulator, by stimulating tactile nerve by stimulation of skin by means of a touch stimulator or the like are collected as pairs with the biomagnetic field signals in respective directions.
A biomagnetic field (hereinafter referred to as cardiac magnetic field) generated from the heart is measured in two directions on the breast side and the back side or in four directions on the breast side, the back side, the right side and the left side of the chest or heart, for example. It is a matter of course that the biomagnetic field generated from the heart may be measured from different directions other than the above directions.
A biomagnetic field (hereinafter referred to cerebral magnetic field) generated from the head (brain) in response to the above stimulation is measured in two directions on the front side and the rear side of the head or brain or in four directions on the right side and the left side of the front side head and the right side and the left side of the rear side head of the head or the brain or in five directions on the right side and the left side of the front side head, on the right side and the left side of the rear side head and on the top of the head or the brain. It is a matter of course that the biomagnetic field generated from the brain may be measured from different directions other than the above directions.
t is time variable. In the orthogonal coordinates (x, y, z), x and y are coordinates or coordinate position where each sensor constituting the sensor array is disposed and a plane parallel to a plane tangential to the surface of the living body is an xy plane, an axis perpendicular to a plane tangential to the surface of the living body being z.
Waveforms of a biomagnetic field measured in many different directions are subjected to the following processing for each direction. When living-body signals periodically generated are measured and collected as pairs with biomagnetic field signals, a time axis of waveforms Wm (t) (m=1, 2, . . . , M) of the living-body signals measured in a plurality of directions of m=1, 2, . . . , M is subjected to conversion Tm (m=1, 2, . . . , M) so that the time axis of the waveforms Wm(t) has a common origin (t=0) where a time variable is t. A time axis of waveforms Fm (m=1, 2, . . . , M) of the biomagnetic field signals paired with the living signals Wm (t) is subjected conversion Tm (m=1, 2, . . . , M). When the synchronous signals synchronizing with the start of application of a stimulation signal are collected as pairs with the biomagnetic field signals, the time axis of waveforms Fm (m=1, 2, . . . , M) of the biomagnetic field signals measured in a plurality of directions of m=1, 2, . . . , M is subjected to conversion Tmxe2x80x2 (m=1, 2, . . . , M) so that the time axis of the waveforms Fm has a common origin (t=0) at times that the synchronous signals are collected. The conversions Tm and Tmxe2x80x2 (m=1, 2, . . . , M) are conversion that the time axis is moved in parallel.
The waveforms of the biomagnetic field (cardiac magnetic field or cerebral magnetic field) measured in the plurality of directions and having the common origin (t=0) are subjected to the following operation processing.
When a magnetic field component Bz (x, y, t) vertical to the plane tangential to the surface of the living body is measured as the biomagnetic field, a variation ∂Bz(x, y, t)/∂x in the x direction and a variation ∂Bz(x, y, t)/∂y in the y direction of the vertical magnetic field component Bz(x, y, t) are calculated and a root sum square, that is, the intensity of a two-dimensional magnetic field vector I (x, y, t) (hereinafter referred to as vector intensity) and the angle xcex8 (x, y, t) thereof are calculated in accordance with equations 1 and 2:
I(x, y, t)={square root over ( )}{(∂Bz(x, y, t)/∂x)2+(∂Bz(x, y, t)/∂y)2}xe2x80x83xe2x80x83(1) 
xcex8 (x, y, t)=xe2x88x92tanxe2x88x921 {(xe2x88x92∂Bz(x, y, t)/∂x)/(∂Bz(x, y, t)/∂y)}xe2x80x83xe2x80x83(2) 
When tangential components (components parallel to a plane tangential to the surface of the living body) Bx and By of a magnetic field generated from the living body is measured, a vector intensity I(x, y, t) and a angle xcex8 (x, y, t) thereof are calculated from a root sum square of the tangential components Bx and By in accordance with equations 3 and 4.
I(x, y, t)={square root over ( )}{(Bx (x, y, t))2+(By (x, y, t))2}xe2x80x83xe2x80x83(3) 
xcex8 (x, y, t)=xe2x88x92tanxe2x88x921 {xe2x88x92Bz(x, y, t)/By(x, y, t)}xe2x80x83xe2x80x83(4) 
Next, a maximum vector intensity Imax(xi, yj, t) and a angle xcex8 (xi, yj, t) thereof at individual time of measured biomagnetic field (cardiac magnetic field or cerebral magnetic field) are calculated. The vector intensity I(x, y, t) is maximum at an i-th x coordinate position and a j-th y coordinate position, that is, at a channel (i, j) of the sensor at time t. The calculated maximum vector intensity Imax(xi, yj, t) and the angle xcex8 (xi, yj, t) thereof at individual time t are displayed for a time variable t. This displayed plots are named a time-intensity plot (t-Imax) and a time-angle plot (t-xcex8), respectively.
As a result of the above, the time-intensity plot (t-Imax) and the time-angle plot (T-xcex8) are obtained from waveforms of the biomagnetic field (cardiac magnetic field or cerebral magnetic field) signals measured in a plurality of directions and having the common origin (t=0). Consequently, the time-intensity (t-Imax) and the time-angle plot (T-xcex8) can be displayed for comparison for each measurement side of the biomagnetic field.
Further, positions (xi, yj) of all the sensors obtained from waveforms of the biomagnetic field (cardiac magnetic field or cerebral magnetic field) signals measured in a plurality of directions and having the common origin (t=0), that is, the vector intensity I(x, y, t) and the angle (x, y, t) thereof at all the channels can be displayed in the same display screen. This displayed plot is named a time-angle intensity plot (t-. . . I). In this display, the angle .(x, y, t) is plotted for a time variable t and the vector intensity I(x, y, t) is displayed while plotted color, a shade of the plotted color or a magnitude of a plotted mark is changed in accordance with the vector intensity I(x, y, t).
As described above, conduction process of electro-physiological excitation can be quantified and displayed by measuring the cardiac or cerebral magnetic field in the plurality of directions without presumption of a dipole and display of many iso-magnetic field maps.
According to the biomagnetic field measuring apparatus of the present invention, since the vector intensity and the angle thereof are used, conduction process of electro-physiological excitation can be quantified and disease and abnormality for each person can be grasped objectively and quantitatively without presumption of a dipole (magnetic field source) by solving an inverse problem and display of many iso-magnetic field maps.